Typical examples of applications in which the novel microbattery disclosed herein become an enabling technology include the newest generations of disposable, self-contained, Lab-On-a-Chip (LOC) diagnostic devices and the newest designs for disposable “smart” contact lenses (i.e., contact lenses which can change their optical properties in response to patient-generated stimuli. Although the specific circuitry, chemistry and/or function of these types of devices are proprietary, it can be deduced that some electronic circuitry must be embedded in the device and that an on-board microbattery will power its logic and mechanical function.
These two exemplary applications, although widely different in their market goals and constructions, generally share similar microbattery technology. Specifically, both the anode and the cathode of the microbattery are provided in alternating and interdigitated fashion, as shown in FIG. 1, in order to provide the maximum surface area exposure per unit area of available space within the microbattery. These two exemplary applications also both connect the microbattery to micro-logic electronic circuits (“control electronics”) which, in turn, switch logic states to control operation of the device (e.g., to control the operation of sensors, to control the movement of fluids, etc. within the device). And, most importantly to the present invention, these two exemplary applications both require an electrolyte to activate the microbattery in order to provide the necessary electrical power to the control electronics.
However, there is a subtle, but important, differentiation between these two exemplary applications. More particularly, in the case of the LOC, the electrolyte for the microbattery may be a body fluid (“biofluid”) that contains some level of a specific marker that also affects the reactivity of the biofluid (i.e., the performance of the biofluid) as the electrolyte for the microbattery. Note that the function of the LOC is to analyze the body fluid by monitoring for this change. This body fluid is, therefore, commonly referred to as the analyte in the case of the LOC. By way of example but not limitation, it is believed that glucose in the blood decreases the efficiency of the blood to act as an electrolyte. So if a drop of blood from a patient with high glucose levels is placed into the microbattery as the activating electrolyte, the power generated by the microbattery is lower than the output from a microbattery where the drop of reference blood electrolyte is low in glucose. Thus, by observing the power generated by the microbattery, the LOC device is able to analyze the glucose marker in the blood (the analyte). So in such an LOC device, the analyte (i.e., the blood) is also the electrolyte for the microbattery.
In the case of the “smart” contact lens, it is possible to vary the optical characteristics (e.g., magnification strength) of a smart contact lens where the smart contact lens is a composite of two or more lenses that define an optical cavity, and where the optical cavity contains an activatable material (e.g., a liquid crystal) which is capable of assuming at least two different optical states, and where the optical state of the activatable material is a function of a voltage applied across the activatable material. Thus, by changing the voltage applied across the activatable material, the optical properties (e.g., magnification strength) of the smart contact lens can be varied. The voltage source for the smart contact lens is an on-board microbattery consisting of an anode and a cathode made from biocompatible materials and an electrolyte (e.g., a biofluid, saline, etc.). In the case of the smart contact lens, the electrolyte is not an analyte per se (since it is not being tested for the presence of a target), but rather is a fluid (e.g., a biofluid, saline, etc.) which is recruited solely for the purpose of serving as the electrolyte activator of the microbattery.
The micro-electronic circuits (“control electronics”) used in biocompatible electronic devices (e.g., LOC's, smart contact lenses, etc.) are typically a collection of thin-film resistors, ceramic micro-capacitors, semiconductors, and thin-film conductors. The power requirements of the biocompatible electronic device (e.g., an LOC or smart contact lens) can be assumed be typical of the current power requirements of a medical implant such as a cochlear implant, which is 30 μW/mm2 from a 1 mm thick film or a Power Density Factor of 3 μW cm−2 μm−1 (see Advances in Electronics Vol 2014, Article ID 981295, 21 pages “Advances in Microelectronics for Implantable Medical Devices” Andreas Demosthenous). By way of example but not limitation, for the lower demand requirements of the smart contact lens application, it is estimated that the power density factor for a demand of 3 μA at a minimum of 1.5 V from a microbattery having an area of approximately 0.1 cm2 and having a maximum stacked film thickness of about 45 μm yields a Power Density Factor of 1 μW cm−2 μm−1. If the wear time of the smart contact lens is presumed to be 18 hours with a 25% duty cycle, the Energy Density Factor is 4.5 μWhr cm−2 μmμ1.
The plot shown in FIG. 2 illustrates the power density-energy density relationship of a number of commercial devices, including four commercial microbatteries (see J. H Pikul, H. G. Zhang, J. Cho, P. V. Braun, and W. P. King, “High-power lithium ion microbatteries from interdigitated three-dimensional bicontinuous nanoporous electrodes,”Nature Communications, vol 4, April 2013). The specifications shown in FIG. 2 indicate that the four commercial microbatteries shown in FIG. 2 are poor candidates for a smart contact lens application. Furthermore, the contents of these four commercial microbatteries are not biocompatible, and even the best packaging for these four commercial microbatteries is subject to dangerous failures from the flammable components which are contained under pressure.
Research into “biocompatible” microbatteries shows that, although their biocompatible components make them safer for use in the body, they tend to suffer from a performance output which is 10-20 times lower than the four commercial microbatteries shown in FIG. 2 (see E.F. Garay, “Biofluid-activated microbattery for disposable microsystems” Masters Thesis, University of Florida. 2013). For example, data from a biocompatible microbattery having aluminum-silver oxide (Al—AgO) electrodes and using a biofluid (human tear drops) as the electrolyte are shown in the table below. However, the test load on the biocompatible Al—AgO microbattery was 3 times higher, the active film of the biocompatible Al—AgO microbattery was ineffectively thin, and the duty cycle was a continuous drain on the microbattery—all factors that yield lower performance than could be made possible with improvements in material choices, battery thickness, and switching improvements between duty cycles.
PowerEnergyDensityDensityAnode-μW cm−2μW h cm−2Predictedcathodeelectrolyteμm−1μm−1run timeAl—AgOUrine352045 minAl—AgOBlood451320 minAl—AgOSaliva221230 min
Similar performance has been achieved with biocompatible microbatteries using magnesium-silver chloride electrodes or magnesium-copper chloride electrodes that can deliver 18 μWhr cm2 μm−1 in a microbattery (see Firas Sammoura, Ki Banf Lee, and Liwei Lin “Water-activated disposable and long shelf-life microbatteries” Sensors and Actuators A 111 (2004) 79-86). The article of Firas Sammoura provides a number of important statements about such biocompatible microbatteries employing magnesium-silver chloride electrodes, or magnesium-copper chloride electrodes, and activated by a biofluid (e.g., water). “Before water was added to their anode-cathode reaction chamber, there is no contact between the electodes and no reaction is expected such that these microbatteries could have a long shelf life. When water is added into the system, surface tension force drives it to fill the reaction chamber. The chemicals dissolve to produce electricity.” With a 5× reduction to the test load and a change to the duty cycle from a continuous drain on the battery to, for example, 20% active time, then a 25 hour microbattery could be realized from either the aluminum-silver oxide or magnesium-silver chloride electrodes or magnesium-copper chloride electrodes. But only if the battery's electrode reaction chamber (“electrode chamber”) remains dry before the electrolyte (i.e., the biofluid) is introduced into the electrode chamber. It is well known that primary batteries using alkaline-derived electrolytes experience a self-discharge rate of 10% per year of storage. It is less well known, however, that the discharge rate for batteries with less-reactive biocompatible electrodes (e.g., Al—AgO, chloride electrodes or magnesium-copper chloride electrodes) would increase linearly with the decrease in electrochemical reactivity—which could increase the self-discharge rate another 10% per year. In addition, powered electrical circuits typically experience another 20% current leakage loss per year. Thus, nearly half of the battery power is typically lost during just a single year of storage.
In addition to the foregoing, in the case of the smart contact lens, it is the common practice to cover the smart contact lens with a soft hydrogel “comfort” layer and to store the smart contact lens in saline, which would initiate electrode activity and “start the clock” of storage power loss at the time of packaging.
Thus it will be seen that improvements are needed in devices (e.g., LOCs, smart contact lenses, etc.) which must incorporate biocompatible microbatteries therein, wherein the microbatteries are intended to employ biofluids as the activating electrolyte and wherein the devices must have a long shelf life prior to insertion into the body in order to make such devices truly enabling for use in the body.